No. 22 Article 2/September 4, 2009

Soybean Aphids Reach Economic Levels in Some Illinois Soybean Fields, Most Notably in Southern Counties

After a relatively quiet summer with soybean aphids across Illinois and much of the north-central region, soybean aphid densities have begun to increase significantly in some fields. Jim Morrison, crop systems extension educator, Rockford Extension Center, indicated that in a sentinel plot in Lee County, 13 of 20 plants examined had 250 to 500 aphids per plant. Both soybean varieties were at the R6 stage of plant development (full seed). Jim also noted the presence of lady beetle adults and larvae in the plot.

In contrast to previous years, this season reports of soybean aphids are more numerous in southern Illinois. Robert Bellm, crop systems extension educator, Edwardsville Extension Center, has observed numerous soybean fields heavily infested with soybean aphids in the past week in Madison County near St. Jacob. Soybean fields with the worst infestations were those planted late or double-cropped.


Soybean aphids on stems, pods, and leaves in a field near St. Jacob, IL, Madison County, August 31, 2009 (Courtesy of Robert Bellm).


Soybean aphids on a soybean leaflet from a field near St. Jacob, IL, Madison County, August 31, 2009 (Courtesy of Robert Bellm).

Because of the late planting in many areas of southern Illinois, some fields have not reached R6. If fields are less mature than R6, they may still benefit from a rescue treatment if an average of 250 aphids per plant is found, 80% of plants are infested, aphid numbers are increasing, and few predators are observed. Keep in mind that actual yield loss does not typically occur unless aphid densities reach 675 aphids per plant. This is the economic injury level. Yield benefits of an insecticide treatment for soybean aphids beyond the R5 stage of plant development have proven inconsistent. Fields should continue to be scouted very thoroughly over the next several weeks and the appropriate management decision made on a field-by-field basis.--Mike Gray

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